Known UV-stabilizers for polymers mostly are derived from the substance classes of triazines, benzotriazoles, benzophenones, sterically hindered amines (HALS=hindered amine light stabilizers) and many more. A list can be found e.g. in DE-A-101 35 795. UV-stabilized, oriented polyester films are also known and the particular activity of stabilizers from the class of triazines for oriented films has also been described (see e.g. WO 98/06575).
WO 98/06575 also points to the varying suitability within the class of triazines and to the commercially available TINUVIN® 1577 (=2-(4,6-diphenyl-1,3,5 triazine-2-yl)-5-(hexyl)oxyphenol, manufactured by Ciba SC, CH) which is particularly suitable.
Further DE-A-101 35 795 describes differences within the group of triazines and proposes to use a number of new as well as known triazines for films. This document also mentions polyester films, however, examples have only been carried out for polyolefin films. Further triazines with bisphenyl units are described as suitable (exemplary compounds a/b/c/d/e/f/g) and the commercially available TINUVIN® 1577 (compound j) is described as particularly suitable. Moreover combinations with other UV-stabilizers are considered, among them compounds from the HALS class. Due to the number of possible variations the total number of potentially suitable stabilizers mentioned in formulas amounts to several thousands. Transfer from polyolefins to (particularly aromatic) polyesters does not appear feasible because the UV absorption and the processing conditions are distinctly different and stabilizers correspondingly must have other properties.
DE-A-101 35 795 as well as WO 98/06575 mention TINUVIN® 1577 as particularly suited and preferred stabilizer.
Despite of its advantages TINUVIN® 1577 has considerable drawbacks, especially in the processing of oriented polyester films. In the melt, similarly to other non-polymeric UV stabilizers, TINUVIN® 1577 does not contribute appreciably to the total viscosity. Because it has to be added in amounts ranging from 1 to 5% by weight in order to achieve commercial grade UV stability (in general a minimum of 5 years lifetime under weather of central European climate conditions or, respectively, 2 years Florida test) a considerable drop in viscosity occurs in film production. This drop is undesirable and causes film breaking, stripes and other manufacturing trouble. Further the use of triazine compounds causes odor which is perceived as an annoyance by many. Moreover triazine compounds like TINUVIN® 1577 have a residual absorbance in the visible light spectral range which causes a yellow discoloration of the film.